Starches with various amylose contents (0%–70%) were processed on a twin‐screw extruder equipped with a special slit die rheometer. Relationships between thermomechanical treatment and starch macromolecular degradation are defined, and flow curves are discussed in order to determine the role of moisture content, product temperature, and mechanical energy on melt viscosity. The viscous behavior is described by a power‐law expression. Viscosity is more sensitive to moisture content and macromolecular degradation at lower amylose contents. Using multiple regression analysis, expressions for the different starches are proposed to describe the influence on the viscosity of amylopectin, which is the macromolecular component with short chain branching. The main differences observed when decreasing the amylose content are a lower viscosity and less pronounced shear thinning. These effects are interpreted in terms of entanglements.